Super absorbent polymer and method for producing same

ABSTRACT

The present invention relates to a super absorbent polymer exhibiting more improved absorption under pressure and liquid permeability, even while basically maintaining excellent centrifuge retention capacity and absorption rate, and a method for producing the same. The super absorbent polymer comprises: a base polymer powder including a first crosslinked polymer of a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups; and a surface crosslinked layer formed on the base polymer powder and including a second crosslinked polymer in which the first crosslinked polymer is further crosslinked via a surface crosslinking agent, wherein the surface crosslinking agent includes at least two compounds having a solubility parameter value (σ) of 12.5 (cal/cm 3 ) 1/2  or more, and wherein at least one of the surface crosslinking agents is an alkylene carbonate-based compound, and the remainder is selected from the group consisting of an alkylene carbonate-based compound and a polyhydric alcohol-based compound.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a national phase entry under 35 U.S.C. § 371of International Application No. PCT/KR2017/011853, filed on Oct. 25,2017, which claims priority to Korean Patent Application No.10-2016-0178406, filed on Dec. 23, 2016, the disclosures of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a super absorbent polymer exhibitingmore improved absorption under pressure and liquid permeability, evenwhile basically maintaining excellent centrifuge retention capacity andabsorption rate, and a method for producing the same.

BACKGROUND

Super absorbent polymer (SAP) is a synthetic polymer material capable ofabsorbing moisture from about 500 to about 1,000 times its own weight,and each manufacturer has denominated it as different names such as SAM(Super Absorbency Material), AGM (Absorbent Gel Material) or the like.Such super absorbent polymers started to be practically applied insanitary products, and now they are widely used for preparation ofhygiene products such as paper diapers for children or sanitary napkins,water retaining soil products for gardening, water stop materials forthe civil engineering and construction, sheets for raising seedling,fresh-keeping agents for food distribution fields, materials forpoultice or the like.

In most cases, these super absorbent polymers have been widely used inthe field of hygienic materials such as diapers or sanitary napkins.

In most cases, these super absorbent polymers have been widely used inthe field of hygienic materials such as diapers or sanitary napkins. Insuch hygienic materials, the super absorbent polymer is generallycontained in a state of being spread in the pulp. In recent years,however, continuous efforts have been made to provide hygienic materialssuch as diapers having a thinner thickness. As a part of such efforts,the development of so-called pulpless diapers and the like in which thecontent of pulp is reduced or pulp is not used at all is being activelyadvanced.

As described above, in the case of hygienic materials in which the pulpcontent is reduced or the pulp is not used, a super absorbent polymer iscontained at a relatively high ratio and these super absorbent polymerparticles are inevitably contained in multiple layers in the hygienicmaterials. In order for the whole super absorbent polymer particlescontained in the multiple layers to absorb liquid such as urine moreefficiently, not only the super absorbent polymer needs to basicallyexhibit high centrifuge retention capacity and absorption rate, but alsoit needs to exhibit more improved liquid permeability and absorptionunder pressure.

That is, the super absorbent polymer should exhibit more improved liquidpermeability, so the super absorbent polymer particles of the surfacelayer which first comes in contact with the liquid such as urine areabsorbed and allow to pass the remaining liquid quickly. It becomespossible to absorb such remaining liquid effectively and quickly by thesuper absorbent polymer particles of the subsequent layer. Further, thesuper absorbent polymer must exhibit a further improved absorption underpressure, whereby the super absorbent polymer can exhibit a stablecentrifuge retention capacity even when the weight of the wearer isapplied in the thinner hygienic material.

However, it is known that the basic centrifuge retention capacity andabsorption rate of the super absorbent polymer, and the liquidpermeability and absorption under pressure thereof have trade-offrelation with each other, and therefore it is very difficult toexcellently maintain these properties together. That is, the basiccentrifuge retention capacity and absorption rate of the super absorbentpolymer is improved as the overall crosslinking density of the polymeris lower, whereas the liquid permeability and absorption under pressuretend to improve as the surface crosslinking density of the polymerincreases and the surface strength of the polymer particles increases.It is a fact that there was a limit in excellently exhibiting theseproperties together.

Conventionally, in an attempt to excellently exhibit these propertiestogether, it has been suggested that the polymerization is carried outin the direction of imparting porosity in order to increase the surfacearea of the super absorbent polymer while lowering the internalcrosslinking density of the super absorbent polymer, and the surfacecrosslinking is performed in the direction of increasing the surfacecrosslinking thickness and surface crosslinking density. However, sinceit is not easy to control the penetration depth of the surfacecrosslinking agent in the surface crosslinking process, there was aproblem that the surface crosslinked layer is hardly formed with uniformthickness and/or density. As a result, there was difficult tosufficiently improve the liquid permeability and the absorption underpressure even in the case of increasing the thickness and thecrosslinking density of the surface crosslinked layer.

Technical Problem

It is one object of the present invention to provide a super absorbentpolymer exhibiting more improved absorption under pressure and liquidpermeability, even while basically maintaining excellent centrifugeretention capacity and absorption rate, and a method for producing thesame.

Technical Solution

The present invention provides a super absorbent polymer comprising: abase polymer powder including a first crosslinked polymer of awater-soluble ethylenically unsaturated monomer having at leastpartially neutralized acidic groups; and a surface crosslinked layerformed on the base polymer powder and including a second crosslinkedpolymer in which the first crosslinked polymer is further crosslinkedvia a surface crosslinking agent,

wherein the surface crosslinking agent includes at least two compoundshaving a solubility parameter value (σ) of 12.5 (cal/cm³)^(1/2) or more,

wherein at least one of the surface crosslinking agents is an alkylenecarbonate-based compound, and the remainder is selected from the groupconsisting of an alkylene carbonate-based compound and a polyhydricalcohol-based compound, and

wherein the permeability measured and calculated by the method of thefollowing Equation 1 is 10 to 35 seconds:Permeability (sec)=T _(S) −T ₀  [Equation 1]

wherein,

T_(S) (unit: sec) means the time required for allowing a 0.9% saline(NaCl) solution to permeate a saline-absorbed super absorbent polymerunder a load of 0.3 psi, wherein the saline-absorbed super absorbentpolymer is prepared by swelling 0.2 g of super absorbent polymer withthe 0.9% saline solution for 30 minutes, and

T₀ (unit: sec) means the time required for allowing the 0.9% salinesolution to permeate under the load of 0.3 psi in the absence of thesaline-absorbed super absorbent polymer.

The present invention also provides a method for producing a superabsorbent polymer comprising the steps of:

performing crosslinking polymerization of a water-soluble ethylenicallyunsaturated monomer having at least partially neutralized acidic groupsin the presence of an internal crosslinking agent to form a hydrogelpolymer containing a first crosslinked polymer;

drying, pulverizing and classifying the hydrogel polymer to form a basepolymer power; and

heat-treating and surface-crosslinking the base polymer powder in thepresence of a surface crosslinking agent to form a super absorbentpolymer particle,

wherein the surface crosslinking agent includes at least two compoundshaving a solubility parameter value (σ) of 12.5 (cal/cm³)^(1/2) or more,

wherein at least one of the surface crosslinking agents is an alkylenecarbonate-based compound, and the remainder is selected from the groupconsisting of an alkylene carbonate-based compound and a polyhydricalcohol-based compound, and

wherein the surface-crosslinking step includes a first reaction step inwhich the reaction is performed at a maximum reaction temperature of170° C. to 190° C. for 3 to 10 minutes and a second reaction step inwhich the reaction is performed at a maximum reaction temperature of190° C. to 220° C. for 20 to 40 minutes.

Hereinafter, a super absorbent polymer according to a specificembodiment of the present invention and a production method thereof willbe described in detail. However, this is merely presented as an exampleof the present invention, and will be apparent to those skilled in theart that the scope of the present invention is not limited to theseembodiments, and various modifications can be made to the embodimentswithin the scope of the present invention.

In addition, unless stated otherwise throughout this specification, theterm “comprises” or “contains” refers to including any constituentelement (or constituent component) without particular limitation, and itcannot be interpreted as a meaning of excluding an addition of otherconstituent element (or constituent component).

Meanwhile, according to one embodiment of the invention, there isprovided a super absorbent polymer comprising:

a base polymer powder including a first crosslinked polymer of awater-soluble ethylenically unsaturated monomer having at leastpartially neutralized acidic groups; and

a surface crosslinked layer formed on the base polymer powder andincluding a second crosslinked polymer in which the first crosslinkedpolymer is further crosslinked via a surface crosslinking agent,

wherein the surface crosslinking agent includes at least two compoundshaving a solubility parameter value (σ) of 12.5 (cal/cm³)^(1/2) or more,

wherein at least one of the surface crosslinking agents is an alkylenecarbonate-based compound, and the remainder is selected from the groupconsisting of an alkylene carbonate-based compound and a polyhydricalcohol-based compound, and

wherein the permeability measured and calculated by the method of thefollowing Equation 1 is 10 to 35 seconds:Permeability (sec)=T _(S) −T ₀  [Equation 1]

wherein,

T_(S) (unit: sec) means the time required for an amount of a 0.9% saline(NaCl) solution to permeate a saline-absorbed super absorbent polymerunder a load of 0.3 psi, wherein the saline-absorbed super absorbentpolymer is prepared by swelling 0.2 g of super absorbent polymer withthe 0.9% saline solution for 30 minutes, and

T₀ (unit: sec) means the time required for the amount of the 0.9% salinesolution to permeate under the load of 0.3 psi in the absence of thesaline-absorbed super absorbent polymer.

The super absorbent polymer of one embodiment is prepared, for example,by using at least two surface crosslinking agents satisfying apredetermined solubility parameter value range, having differentreactivity, at least one of which is an alkylene carbonate-basedcompound. In addition, such a super absorbent polymer can be produced byproceeding the surface crosslinking step to which the surfacecrosslinking agent is applied under constant temperature rising andreaction conditions, as will be described in more detail below.

As a result of continuous studies and experiments, the present inventorshave found that as the surface crosslinking step proceeds under thesepredetermined surface crosslinking agents and conditions, thepenetration depth of the surface crosslinking agent can be uniformlycontrolled, and the degree of crosslinking reaction can be uniformlycontrolled during surface crosslinking. This is predicted to be becauseby using a surface crosslinking agent satisfying the above solubilityparameter value range, it is possible to maintain the uniform coatingeffect of the surface crosslinking agent on the base polymer powder andto uniformly control the penetration depth, despite the evaporation ofthe solvent during surface crosslinking and the change in theconcentration of the surface crosslinking agent resulting therefrom. Inaddition, It is thought that as two or more kinds of surfacecrosslinking agents having different reactivity and structure are usedand the surface crosslinking is carried out under constant surfacecrosslinking conditions, the surface crosslinking reaction can be causeduniformly despite the irregular shape of the base polymer powder, etc.,thereby forming a surface crosslinked layer having uniform thickness andcrosslinking density on the base polymer powder.

It has been found that due to the formation of such a uniform surfacecrosslinked layer, the surface crosslinked layer is formed with arelatively thin thickness, and thereby even if the basic centrifugeretention capacity and absorption rate are slightly decreased, theliquid permeability and the absorption under pressure can be improvedwith a width larger than previously known. Such improved liquidpermeability can be confirmed by the low permeability measured andcalculated according to the Equation 1.

Therefore, the super absorbent polymer of one embodiment can exhibitmore improved absorption under pressure and liquid permeability, evenwhile basically maintaining excellent centrifuge retention capacity andabsorption rate, and thus can be effectively used for various kinds ofhygienic materials, especially hygienic materials or the like havingreduced pulp content.

In the super absorbent polymer of one embodiment, the base polymerpowder may have the component and composition equivalent to those of thebase polymer powder of a general super absorbent polymer. Such basepolymer powder may include a first crosslinked polymer of an unsaturatedmonomer having at least partially neutralized acidic groups, such asacrylic acid and its salt, and can be produced according to theproduction method of another embodiment described below. Therefore,additional description of the constitution of the base polymer powderwill be omitted.

Further, in the super absorbent polymer of one embodiment, the surfacecrosslinked layer may include a second crosslinked polymer in which thefirst crosslinked polymer of the base polymer powder is furthercrosslinked via two or more specific surface crosslinking agents.

As the surface crosslinking agent for forming such a surface crosslinkedlayer, two or more compounds having a solubility parameter value (σ) of12.5 (cal/cm³)^(1/2) or more, or 12.5 to 18.0 (cal/cm³)^(1/2), or 12.6to 17.0 (cal/cm³)^(1/2) can be used. The solubility parameter value (σ)is a parameter that defines the polarity of a compound, and can bedetermined with reference to the solubility parameter valueδ(cal/cm³)^(1/2) reported in Polymer Handbook, 3rd edition (WileyInterscience Corp.) VII-527-539. Further, in the case of a compound notincluded in the relevant list, it can be calculated with the numericalvalue δ (cal/cm³)^(1/2) derived by replacing Holley cohesive energyconstant reported in the same volume VII-525 in Small's Equation in thesame volume VII-524. The solubility parameter values of representativecompounds determined or calculated according to the above method aresummarized in Table 1 below:

TABLE 1 Solubility parameter Compound value ((cal/cm³)^(1/2)) Ethylenecarbonate 14.5 Propylene carbonate 13.3 Propylene glycol 12.6 Glycerol16.5 Ethylene glycol diglycidyl ether 10.2 1,3-butanediol 11.6

As already mentioned above, it is considered that as two or more surfacecrosslinking agents having a solubility parameter value (σ) of 12.5(cal/cm³)^(1/2) or more are used, but at least one of them uses analkylene carbonate-based compound and the remainder uses an alkylenecarbonate-based compound and a polyhydric alcohol-based compound havingdifferent reactivity and structure, the uniform coating effect and thepenetration depth of the surface crosslinking agent can be achieveddespite the uneven particle shape of the base polymer powder and thelike. As a result, a surface crosslinked layer having a uniformthickness and a crosslinked density can be formed on the super absorbentpolymer particles of one embodiment, thereby further improving theliquid permeability and the absorption under pressure.

On the other hand, ethylene carbonate or propylene carbonate may be usedas the alkylene carbonate-based compound having a solubility parametervalue of 12.5 (cal/cm³)^(1/2) or more, and at least one of the surfacecrosslinking agents may be used as such ethylene carbonate or propylenecarbonate.

As the polyhydric alcohol-based compound having a solubility parametervalue of 12.5 (cal/cm³)^(1/2) or more, ethylene glycol, propyleneglycol, glycerol, polyglycerol, sorbitol or pentaerythritol may be used.In consideration of the degree of reaction, etc., more suitably,propylene glycol or glycerol may be used. Such a polyhydricalcohol-based compound may be used as the remaining surface crosslinkingagent except for at least one alkylene carbonate-based compound.

Meanwhile, the above-mentioned super absorbent polymer can exhibitexcellent liquid permeability, and this excellent liquid permeabilitycan be defined by the permeability of 10 to 35 seconds or 20 to 30seconds measured and calculated by the method of the Equation 1:Permeability (sec)=T _(S) −T ₀  [Equation 1]

wherein:

T_(S) (unit: sec) means the time required for allowing a 0.9% saline(NaCl) solution to permeate a saline-absorbed super absorbent polymerunder a load of 0.3 psi, wherein the saline-absorbed super absorbentpolymer is prepared by swelling 0.2 g of super absorbent polymer withthe 0.9% saline solution for 30 minutes, and

T₀ (unit: sec) means the time required for allowing the 0.9% salinesolution to permeate under the load of 0.3 psi in the absence of thesaline-absorbed super absorbent polymer.

The permeability is an index representing how well a saline solution(0.9% NaCl aqueous solution) permeates the swollen super absorbentpolymer. This is evaluated by measuring the time taken for 0.9% salinesolution to permeate after swelling 0.2 g of the super absorbent polymerpowder for 30 minutes and then applying a load of 0.3 psi, in accordancewith the method described in the literature (Buchholz. F. L. and Graham.A. T., “Modern Super absorbent polymer Technology.” John Wiley & Sons(1998), page 161). A more detailed method of measuring the permeabilitywill be described in detail in the embodiments described below.

Further, the super absorbent polymer of one embodiment may have a fixedheight absorption (FHA) under Capillary Pressure of Loaded SAP bed of 20to 30 g/g, or 21.5 to 28.5 g/g, or 24.5 to 28.5 g/g. Such high FHA valuecan define an excellent suction force under pressure (capillaryabsorption capacity) of the super absorbent polymer. Such FHA can bemeasured and calculated by the method described in column 14/example ofU.S. Pat. No. 7,175,910.

In addition, the super absorbent polymer can basically exhibit excellentcentrifuge retention capacity and absorption rate. More specifically,the super absorbent polymer may have a centrifuge retention capacity(CRC) for a physiological saline solution (0.9 wt % aqueous sodiumchloride solution) for 30 minutes of 25 g/g to 36 g/g, or 28 g/g to 36g/g, or 28 g/g to 32 g/g.

In this case, the centrifuge retention capacity (CRC) can be calculatedby the following Equation 3 after absorbing the super absorbent polymerto a physiological saline solution over 30 minutes:CRC(g/g)={[W ₂(g)−W ₁(g)−W ₀(g)]/W ₀(g)}

wherein:

W₀ (g) is an initial weight (g) of the super absorbent polymer, W₁ (g)is the weight of the device not including the super absorbent polymer,measured after immersing and absorbing the same into a physiologicalsaline solution for 30 minutes and then dehydrating the same by using acentrifuge at 250 G for 3 minutes, and W₂ (g) is the weight of thedevice including the super absorbent polymer, measured after immersingand absorbing the super absorbent polymer into a physiological salinesolution at room temperature for 30 minutes and then dehydrating thesame by using a centrifuge at 250 G for 3 minutes.

Additionally, the super absorbent polymer of one embodiment may have avortex removal time of 20 to 50 seconds, or 35 to 50 seconds, or 38 to45 seconds, together with the above-described centrifuge retentioncapacity and the like. Such a vortex removal time means the time duringwhich a vortex of the liquid disappears due to fast absorption when thesuper absorbent polymer is added to the physiological saline andstirred.

For example, after the super absorbent polymer is added to aphysiological saline (0.9 wt % NaCl solution) under stirring, the vortexremoval time can be calculated by a method of measuring the amount oftime until a vortex of the liquid caused by the stirring disappears anda smooth surface is formed.

Meanwhile, the super absorbent polymer of one embodiment described abovecan be typically obtained by performing a crosslinking polymerization ofa water-soluble ethylenically unsaturated monomer having at leastpartially neutralized acidic groups, such as a mixture of acrylic acidand its sodium salt in which at least some carboxylic acid has beenneutralized with sodium salt or the like, in the presence of an internalcrosslinking agent. More specifically, the super absorbent polymer canbe obtained by performing a crosslinking polymerization of said monomerin the presence of an internal crosslinking agent to prepare a basepolymer powder, and then surface-crosslinking the base polymer powder inthe presence of a specific surface crosslinking agent as describedabove, under certain crosslinking conditions.

In particular, by controlling the kind of the surface crosslinking agentand the surface crosslinking conditions, it is possible to obtain asuper absorbent polymer of one embodiment which includes a surfacecrosslinked layer having a uniform thickness and crosslinking densityand exhibits more improved liquid permeability and absorption underpressure even while maintaining excellent centrifuge retention capacityand absorption rate.

In this regard, according to another embodiment of the invention, thereis provided a method for producing a super absorbent polymer comprisingthe steps of: performing crosslinking polymerization of a water-solubleethylenically unsaturated monomer having at least partially neutralizedacidic groups in the presence of an internal crosslinking agent to forma hydrogel polymer containing a first crosslinked polymer; drying,pulverizing and classifying the hydrogel polymer to form a base polymerpower; and heat-treating and surface-crosslinking the base polymerpowder in the presence of a surface crosslinking agent to form a superabsorbent polymer particle, wherein the surface crosslinking agentincludes at least two compounds having a solubility parameter value (σ)of 12.5 (cal/cm³)^(1/2) or more, wherein at least one of the surfacecrosslinking agents is an alkylene carbonate-based compound, and theremainder is selected from the group consisting of an alkylenecarbonate-based compound and a polyhydric alcohol-based compound, andwherein the surface-crosslinking step includes a first reaction step inwhich the reaction is performed at a maximum reaction temperature of170° C. to 190° C. for 3 to 10 minutes and a second reaction step inwhich the reaction is performed at a maximum reaction temperature of190° C. to 220° C. for 20 to 40 minutes.

Hereinafter, one embodiment of the production method of the superabsorbent polymer will be described in detail for each step.

First, in the above-mentioned production method, the hydrogel polymercan be formed by performing crosslinking polymerization of awater-soluble ethylenically unsaturated monomer having at leastpartially neutralized acidic groups in the presence of an internalcrosslinking agent.

In this case, the water-soluble ethylenically unsaturated monomer mayinclude at least one selected from the group consisting of anionicmonomers of acrylic acid, methacrylic acid, maleic anhydride, fumaricacid, crotonic acid, itaconic acid, 2-acryloylethanesulfonic acid,2-methacryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acidor 2-(meth)acrylamido-2-methylpropanesulfonic acid, and their salts;non-ionic, hydrophilic group-containing monomers of (meth)acrylamide,N-substituted (meth)acrylate, 2-hydroxyethyl(meth)acrylate,2-hydroxypropyl(meth)acrylate, methoxypolyethylene glycol(meth)acrylateor polyethylene glycol (meth)acrylate; and amino group-containingunsaturated monomers of (N,N)-dimethylaminoethyl(meth)acrylate or(N,N)-dimethylaminopropyl(meth)acrylamide, and their quaternary product.Among them, acrylic acid or a salt thereof, for example, an alkali metalsalt such as acrylic acid and/or a sodium salt thereof having at leastpartially neutralized acrylic acids can be used, and the use of thesemonomers enables production of a super absorbent polymer having moreexcellent physical properties.

In the case of using acrylic acid and its alkali metal salt as amonomer, the monomer can be used by neutralizing acrylic acid with abasic compound such as caustic soda (NaOH). In this case, a degree ofneutralization of the water-soluble ethylenically unsaturated monomermay be adjusted to 50 to 95%, or 70 to 85%. Within this range, it ispossible to provide a super absorbent polymer having excellentcentrifuge retention capacity without fear of precipitation duringneutralization.

In the monomer mixture containing the water-soluble ethylenicallyunsaturated monomer, the concentration of the water-solubleethylenically unsaturated monomer may be 20 to 60% by weight or 40 to50% by weight based on the total amount of the monomer mixture includingthe respective raw materials and solvents described below, which may beappropriately adjusted in consideration of polymerization time, thereaction conditions and the like. However, when the concentration of themonomer is excessively low, the yield of the super absorbent polymer maybe lowered, which may cause a problem in economic efficiency.Conversely, when the concentration is excessively high, a part of themonomer may precipitate or the pulverization efficiency may be loweredupon pulverization of the polymerized hydrogel polymer, which may causea problem in the process, and the physical properties of the superabsorbent polymer may be deteriorated.

As the internal crosslinking agent for introducing the basiccrosslinking structure into the base polymer powder, any internalcrosslinking agent having a crosslinking functional group conventionallyused for the production of a super absorbent polymer can be used withoutparticular limitation. However, in order to further improve the physicalproperties of the super absorbent polymer by introducing an appropriatecrosslinking structure into the base polymer powder, more specifically,in order to appropriately achieve physical properties of the basepolymer powder described above and thus exhibit an improved liquidpermeability and cake inhibition properties of the super absorbentpolymer, at least one selected from the group consisting of abis(meth)acrylamide having 8 to 12 carbon atoms, a polyolpoly(meta)acrylate having 2 to 10 carbon atoms and a polyolpoly(meth)ally ether having 2 to 10 carbon atoms can be used as theinternal crosslinking agent.

More specific examples of the internal crosslinking agent may include atleast one selected from the group consisting of polyethylene glycoldiacrylate (PEGDA), glycerine diacrylate, glycerin triacrylate,unmodified or ethoxylated trimethylolpropane triacrylate(ethoxylated-TMPTA), hexanediol diacrylate, and triethylene glycoldiacrylate. The internal crosslinking agent can be contained at aconcentration of 0.01 to 0.5% by weight with respect to the monomermixture, thereby crosslinking the polymerized polymer.

In addition, the monomer mixture may further include a polymerizationinitiator that is generally used in the production of a super absorbentpolymer.

Specifically, the polymerization initiator that can be used hereincludes a thermal polymerization initiator or a photo-polymerizationinitiator by UV irradiation, depending on the polymerization method.However, even in the case of using the photo-polymerization method,because a certain amount of heat is generated by the ultravioletirradiation or the like and a certain degree of heat is generatedaccording to the progress of the exothermic polymerization reaction, athermal polymerization initiator may be additionally included.

The photo-polymerization initiator can be used without any limitation inits constitution as long as it is a compound capable of forming aradical by a light such as ultraviolet rays. The photo-polymerizationinitiator, for example, may include at least one selected from the groupconsisting of a benzoin ether, a dialkyl acetophenone, a hydroxylalkylketone, a phenyl glyoxylate, a benzyl dimethyl ketal, an acylphosphine, and an α-aminoketone. Meanwhile, specific examples of theacyl phosphine may include normal lucirin TPO, namely,2,4,6-trimethyl-benzoyl-trimethyl phosphine oxide, or IRGACURE 819,namely, bis(2,4,6-trimethylbenzoyl)-phenyl phosphineoxide).

More various photo-polymerization initiators are well disclosed in “UVCoatings: Basics, Recent Developments and New Application” written byReinhold Schwalm, (Elsevier, 2007), p 115, however thephoto-polymerization initiator is not limited to the above-describedexamples.

The photo-polymerization initiator may be included in a concentration of0.01 to 1.0% by weight based on the monomer mixture. When theconcentration of the photo-polymerization initiator is excessively low,the polymerization rate may become slow, and when the concentration ofthe photo-polymerization initiator is excessively high, the molecularweight of the super absorbent polymer may become small and its physicalproperties may become uneven.

Further, as the thermal polymerization initiator, at least one selectedfrom the group consisting of persulfate-based initiator, azo-basedinitiator, hydrogen peroxide and ascorbic acid can be used.Specifically, examples of the persulfate-based initiators include sodiumpersulfate (Na₂S₂O₈), potassium persulfate (K₂S₂O₈), ammonium persulfate((NH₄)₂S₂O₈) and the like, and examples of the azo-based initiatorinclude 2,2-azobis(2-amidinopropane)dihydrochloride,2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride,2-(carbamoylazo)isobutylonitril,2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride,4,4-azobis-(4-cyanovaleric acid) and the like.

More various thermal polymerization initiators are well disclosed in“Principle of Polymerization” written by Odian, (Wiley, 1981), p 203,however the thermal polymerization initiator is not limited to theabove-described examples.

The thermal polymerization initiator can be included in theconcentration of 0.001 to 0.5% by weight based on the monomercomposition. When the concentration of the thermal polymerizationinitiator is excessively low, the additional thermal polymerizationhardly occurs and thus effects due to the addition of the thermalpolymerization initiator may be insignificant, and when theconcentration of the thermal polymerization initiator is excessivelyhigh, the molecular weight of the super absorbent polymer becomes smalland the physical properties may become uneven.

In addition, the monomer mixture may further include additives such as athickener, a plasticizer, a preservation stabilizer, an antioxidant, asurfactant, and so on, as needed. As the additives to be added, variousadditives widely used in the field of the super absorbent polymer can beused without limitation.

Meanwhile, the raw materials such as the water-soluble ethylenicallyunsaturated monomer, the photo-polymerization initiator, the thermalpolymerization initiator, the internal crosslinking agent, and theadditives can be prepared in the form of a solution which is dissolvedin a solvent.

In this case, any usable solvent can be used without limitation in itsconstitution as long as it can dissolve the above-mentioned components.For example, one or more solvents selected from the group consisting ofwater, ethanol, ethyleneglycol, diethyleneglycol, triethyleneglycol,1,4-butanediol, propyleneglycol, ethyleneglycol monobutylether,propyleneglycol monomethylether, propyleneglycol monomethyletheracetate, methylethylketone, acetone, methylamylketone, cyclohexanone,cyclopentanone, diethyleneglycol monomethylether, diethyleneglycolethylether, toluene, xylene, butylolactone, carbitol, methylcellosolveacetate, and N,N-dimethyl acetamide, and so on may be used alone or incombination with each other.

The solvent may be included in a residual amount of excluding theabove-described components from the total weight of the monomer mixture.

Meanwhile, the method for forming a hydrogel polymer by the thermalpolymerization or photopolymerization of such a monomer composition isnot particularly limited by its constitution as long as it is apolymerization method commonly used in the art.

Specifically, the polymerization process may be largely classified intoa thermal polymerization and a photo-polymerization depending on apolymerization energy source. The thermal polymerization may betypically carried out in a reactor like a kneader equipped withagitating spindles in order to facilitate bubble generation. Incontrast, the photo-polymerization may be carried out in a reactorequipped with a movable conveyor belt. However, the above-describedpolymerization method is an example only, and the present invention isnot limited thereto.

Further, a polymerization temperature of the monomer mixture may becontrolled from about 40° C. to 90° C., thereby effectively inducingvaporization of the volatile organic solvent and polymerization of thehydrogel polymer in which pores are formed.

In this case, means for achieving the polymerization temperature withinthe above-described range is not particularly limited. Heating may beperformed by providing a heating medium or by directly providing a heatsource. The type of the heating medium applicable may be a heated fluidsuch as steam, hot air, hot oil, etc., but is not limited thereto.Further, the temperature of the heating medium provided may be properlyselected in consideration of the means of the heating medium, thetemperature raising speed, and the temperature raising targettemperature. Meanwhile, a heating method using electricity or a heatingmethod using gas may be used as the heat source provided directly, butthe heat source is not limited to these examples.

Further, a polymerization time of the monomer mixture may be controlledfrom 30 seconds to 10 minutes, thereby forming a hydrogel polymer havingan optimized pore structure.

As an example, the hydrogel polymer obtained according to the thermalpolymerization that is carried out in the reactor like a kneaderequipped with a stirring spindle by providing hot air thereto or heatingthe reactor may have a particle size of several centimeters to severalmillimeters when it is discharged from the outlet of the reactor,according to the shape of the stirring spindle equipped in the reactor.Specifically, the size of the obtained hydrogel polymer may varyaccording to the concentration of the monomer composition injectedthereto, the injection speed, or the like, and generally the hydrogelpolymer having a weight average particle size of 2 to 50 mm may beobtained.

Further, as described above, when the photo-polymerization is carriedout in a reactor equipped with a movable conveyor belt, the obtainedhydrogel polymer may be usually a sheet-like hydrogel polymer having awidth of the belt. In this case, the thickness of the polymer sheet mayvary depending on the concentration and the injection speed of themonomer mixture to be injected thereto, but usually, it is preferable tosupply the monomer mixture so that a sheet-like polymer having athickness of about 0.5 to about 5 cm can be obtained. When the monomermixture is supplied to such an extent that the thickness of thesheet-like polymer becomes too thin, it is undesirable because theproduction efficiency is low, and when the thickness of the sheet-likepolymer is more than 5 cm, the polymerization reaction cannot be evenlycarried out over the entire thickness because of the excessivethickness.

In this case, the hydrogel polymer obtained by the above-mentionedmethod may have a water content of 40 to 80% by weight. Meanwhile, the“water content” as used herein means a weight occupied by moisture withrespect to a total amount of the hydrogel polymer, which may be thevalue obtained by subtracting the weight of the dried polymer from theweight of the hydrogel polymer. Specifically, the water content can bedefined as a value calculated by measuring the weight loss due toevaporation of moisture in the polymer in the process of drying byraising the temperature of the polymer through infrared heating. At thistime, the water content is measured under the drying conditionsdetermined as follows: the drying temperature is increased from roomtemperature to about 180° C. and then the temperature is maintained at180° C., and the total drying time is set to 20 minutes, including 5minutes for the temperature rising step.

After the monomers are subjected to a crosslinking polymerization, thebase polymer powder can be obtained through steps such as drying,pulverization, classification, and the like, and through the steps suchas pulverization and classification, the base polymer powder and thesuper absorbent polymer obtained therefrom are suitably produced andprovided so as to have a particle diameter of 150 to 850 μm. Morespecifically, at least 95% by weight or more of the base polymer powderand the super absorbent polymer obtained therefrom has a particlediameter of 150 μm to 850 μm and a fine powder having a particlediameter of less than 150 μm can contained in an amount of less than 3%by weight.

As described above, as the particle diameter distribution of the basepolymer powder and the super absorbent polymer is adjusted within thepreferable range, the super absorbent polymer finally produced canexhibit the above-mentioned physical properties more satisfactorily.

On the other hand, the method of drying, pulverization andclassification will be described in more detail below.

First, when drying the hydrogel polymer, a coarsely pulverizing step maybe further carried out before drying in order to increase the efficiencyof the drying step, if necessary.

A pulverizing machine used here is not limited by its configuration, andspecifically, it may include any one selected from the group consistingof a vertical pulverizer, a turbo cutter, a turbo grinder, a rotarycutter mill, a cutter mill, a disc mill, a shred crusher, a crusher, achopper, and a disc cutter. However, it is not limited to theabove-described examples.

In this case, the coarsely pulverizing step may be carried out so thatthe particle diameter of the hydrogel polymer becomes about 2 mm toabout 10 mm.

Pulverizing the hydrogel polymer into a particle diameter of less than 2mm is technically not easy due to its high water content, andagglomeration phenomenon between the pulverized particles may occur.Meanwhile, if the polymer is pulverized into a particle diameter ofgreater than 10 mm, the effect of increasing the efficiency in thesubsequent drying step may be insignificant.

The hydrogel polymer coarsely pulverized as above or the hydrogelpolymer immediately after polymerization without the coarselypulverizing step is subjected to a drying step. In this case, the dryingtemperature of the drying step may be 50° C. to 250° C. When the dryingtemperature is less than 50° C., it is likely that the drying timebecomes too long or the physical properties of the super absorbentpolymer finally formed is deteriorated, and when the drying temperatureis higher than 250° C., only the surface of the polymer is excessivelydried, and thus it is likely that fine powder is generated during thesubsequent pulverizing step, and the physical properties of the superabsorbent polymer finally formed is deteriorated. Preferably, the dryingcan be carried out at a temperature of 150 to 200° C., more preferablyat a temperature of 160 to 190° C.

Meanwhile, the drying time may be 20 minutes to 15 hours, inconsideration of the process efficiency and the like, but it is notlimited thereto.

In the drying step, the drying method may also be selected and usedwithout being limited by its constitution if it is a method generallyused for drying the hydrogel polymer. Specifically, the drying step maybe carried out by a method such as hot air supply, infrared irradiation,microwave irradiation or ultraviolet irradiation. After the drying stepas above is carried out, the water content of the polymer may be 0.05 to10% by weight.

Subsequently, the dried polymer obtained through the drying step issubjected to a pulverization step.

The polymer powder obtained through the pulverizing step may have aparticle diameter of 150 μm to 850 μm. Specific examples of apulverizing device that can be used to achieve the above particlediameter may include a ball mill, a pin mill, a hammer mill, a screwmill, a roll mill, a disc mill, a jog mill or the like, but the presentinvention is not limited thereto.

Also, in order to control the physical properties of the super absorbentpolymer powder finally commercialized after the pulverization step, aseparate step of classifying the polymer powder obtained after thepulverization depending on the particle diameter may be undergone.Preferably, a polymer having a particle diameter of 150 to 850 μm isclassified and only the polymer powder having such a particle diameteris subjected to the surface crosslinking reaction and finallycommercialized.

On the other hand, after performing the step of forming the base polymerpowder described above, the method for producing the super absorbentpolymer of the one embodiment may include a step of heat-treating andsurface-crosslinking the base polymer powder in the presence of asurface crosslinking agent satisfying a predetermined solubilityparameter value range, at least one of which is an alkylenecarbonate-based compound. The surface crosslinking step may be carriedout through a plurality of reaction steps including a first reactionstep in which the reaction is performed at a maximum reactiontemperature of 170° C. to 190° C. or 180° C. to 188° C. for 3 to 10minutes, or 4 to 8 minutes, and a second reaction step in which thereaction is performed at a higher temperature than that of the firstreaction step, for example, at a maximum reaction temperature of 190° C.to 220° C., or 192° C. to 200° C. for 20 to 40 minutes, or 25 to 35minutes.

By applying such surface crosslinking conditions and the surfacecrosslinking agents described above, the super absorbent polymer cancontain a surface crosslinked layer having more uniform thickness andcrosslinking density, and more improved liquid permeability andabsorption under pressure can be exhibited without greatly lowering thebasic centrifuge retention capacity and the like after formation of thesurface crosslinked layer.

On the other hand, since the kind of the surface crosslinking agent hasalready been described in the above-mentioned surface crosslinking step,additional description thereof will be omitted.

The surface crosslinking agent may be provided in the form of a surfacecrosslinking solution which is dissolved in a solvent. Such surfacecrosslinking solution may include at least one solvent selected from thegroup consisting of water, ethanol, ethylene glycol, diethylene glycol,triethylene glycol, 1,4-butanediol, propylene glycol, ethylene glycolmonobutyl ether, propylene glycol monomethyl ether, propylene glycolmonomethyl ether acetate, methyl ethyl ketone, acetone, methyl amylketone, cyclohexanone, cyclopentanone, diethylene glycol monomethylether, diethylene glycol ethylether, toluene, xylene, butyrolactone,carbitol, methyl cellosolve acetate, and N,N-dimethylacetamide.

Further, the surface crosslinking agent contained in the surfacecrosslinking solution can be used in an amount of 0.01 to 4 parts byweight based on 100 parts by weight of the base polymer powder. Amongsuch surface crosslinking solution, at least one surface crosslinkingagent being an alkylene carbonate-based compound and the remainingsurface crosslinking agent can be used by mixing in a weight ratio of5:1 to 1:5, or 4:1 to 1:4, or 3:1 to 1:3.

Further, the surface crosslinking step can be carried out by includingthe polycarboxylic acid-based polymer together with the surfacecrosslinking agent in the surface crosslinking solution. As thepolycarboxylic acid-based polymer, a polycarboxylic acid-based copolymerdisclosed in Korean Patent Laid-Open Publication No. 2015-0143167(Korean Patent Application No. 2014-0072343) can be representativelyused. Such polymer may be contained in the surface cross-linkingsolution in an amount of 1 part by weight or less, or 0.01 to 0.5 partsby weight, based on 100 parts by weight of the base polymer powder. Thispolymer can be contained in the surface crosslinking solution in anamount of 1 part by weight or less, or 0.01 to 0.5 part by weight basedon 100 parts by weight of the base polymer powder. Due to the additionaluse of these polycarboxylic acid-based polymers, it is possible tofurther improve the liquid permeability and the centrifuge retentioncapacity of the super absorbent polymer.

Moreover, in the surface crosslinking step, the surface crosslinkingsolution may further contain a thickener. If the surface of the basepolymer powder is further crosslinked in the presence of the thickeneras described above, it is possible to minimize the deterioration ofphysical properties even after the pulverization.

More specifically, as the thickener, at least one selected from apolysaccharide and a hydroxy-containing polymer may be used.

Among them, the polysaccharide may be a gum type thickener, a cellulosetype thickener and the like. Specific examples of the gum type thickenerinclude xanthan gum, arabic gum, karaya gum, tragacanth gum, ghatti gum,guar gum, locust bean gum, psyllium seed gum and the like. Specificexamples of the cellulose type thickener include hydroxypropylmethylcellulose, carboxymethyl cellulose, methylcellulose, hydroxymethylcellulose, hydroxyethyl cellulose, hydroxypropyl cellulose,hydroxyethylmethyl cellulose, hydroxymethylpropyl cellulose,hydroxyethylhydroxypropyl cellulose, ethylhydroxyethyl cellulose,methylhydroxypropyl cellulose, and the like. Meanwhile, specificexamples of the hydroxy-containing polymer include polyethylene glycol,polyvinyl alcohol, and the like.

The surface crosslinking solution may further include a thermalinitiator such as Na₂S₂O₅ so that the surface crosslinking reaction mayoccur more smoothly depending on the kind of the surface crosslinkingsolution, and may further include a commercially available discolorationinhibitor.

On the other hand, the surface crosslinking step can be carried out byheat-treating the base polymer powder in the presence of theabove-mentioned surface crosslinking solution. More specifically, thestep of the reaction by such a heat treatment may be carried out througha plurality of reaction steps including a first reaction step in whichthe reaction is performed at a maximum reaction temperature of 170° C.to 190° C., or 180° C. to 188° C. for 3 to 10 minutes, or 4 to 8minutes, and a second reaction step in which the reaction is performedat a higher temperature than that of the first reaction step, forexample, at a maximum reaction temperature of 190° C. to 220° C. or 192°C. to 200° C. for 20 to 40 minutes, or 25 to 35 minutes.

In addition, between these plural reaction steps, for example, beforethe first and second reaction steps, the temperature raising step forreaching the maximum reaction temperature may be further included. Thetemperature raising step before the first reaction step may be carriedout at a temperature raising rate of 1.5° C./min to 3.0° C./min or 2.0°C./min to 2.7° C./min. and the temperature rising step before the secondreaction step (i.e., the temperature rising step between the first andsecond reaction steps) may be carried out at a temperature raising rateof 1.2° C./min to 2.3° C./min or at a temperature raising rate of 1.5°C./min to 2.2° C./min.

As described above, a plurality of reaction steps to which apredetermined maximum reaction temperature and a holding time areapplied and, more suitably, the surface crosslinking step proceeds underthe application of a constant heating rate, the surface crosslinkingreaction can occur more uniformly despite the irregular particle shapeof the base polymer powder and the like. As a result, a surfacecrosslinked layer having a very uniform thickness and a crosslinkingdensity can be formed. Therefore, it is possible to produce a superabsorbent polymer satisfying more improved liquid permeability andabsorption under pressure while maintaining excellent centrifugeretention capacity or absorption rate.

Meanwhile, the temperature raising means for the surface crosslinkingreactions is not particularly limited. The heating can be carried out byproviding a heating medium or directly providing a heating source. Thetype of heat medium that can be used here includes a heated fluid suchas steam, hot air, hot oil, etc., but it is not limited thereto.Further, the temperature of the heating medium to be provided can beappropriately selected in consideration of the means of the heatingmedium, the temperature raising speed, and the temperature raisingtarget temperature. Meanwhile, a heat source to be provided directly mayinclude a heating method using electricity or a heating method usinggas, but is not limited to the above-described examples.

According to the present invention, there can be provided a superabsorbent polymer exhibiting more improved absorption under pressure andliquid permeability, even while basically maintaining excellentcentrifuge retention capacity and absorption rate, and a method forproducing the same.

Such super absorbent polymer is preferably used for hygienic materialssuch as diapers in which the pulp content is reduced or the pulp is notused, thereby exhibiting excellent performance.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, preferred examples are provided for better understanding ofthe invention. However, these Examples are given for illustrativepurposes only and are not intended to limit the scope of the presentinvention thereto.

In the following Examples and Comparative Examples, the physicalproperties of the respective super absorbent polymers (or base polymerpowders) were measured and evaluated by the following methods.

(1) Evaluation of Particle Diameter

The particle diameter of the super absorbent polymer used in Examplesand Comparative Examples was measured in accordance with EDANA (EuropeanDisposables and Nonwovens Association) recommended test method No. WSP220.3.

(2) Centrifuge Retention Capacity (CRC)

The centrifuge retention capacity (CRC) by water absorption capacityunder a non-loading condition was measured for the super absorbentpolymers of Examples and Comparative Examples in accordance with EDANA(European Disposables and Nonwovens Association) recommended test methodNo. WSP 241.3.

That is, W₀ (g, about 0.2 g) of the super absorbent polymer wasuniformly put in a nonwoven fabric-made bag, followed by sealing. Then,the bag was immersed in a physiological saline solution composed of 0.9wt % aqueous sodium chloride solution at room temperature. After 30minutes, water was removed from the bag by centrifugation at 250 G for 3minutes, and the weight W₂ (g) of the bag was then measured. Further,the same procedure was carried out without using the super absorbentpolymer, and then the resultant weight W₁ (g) was measured.

Using the respective weights thus obtained, CRC (g/g) was calculatedaccording to the following Equation 3, thereby confirming the centrifugeretention capacity.CRC(g/g)={[W ₂(g)−W ₁(g)−W ₀(g)]/W ₀(g)}

wherein, W₀ (g) is an initial weight (g) of the super absorbent polymer,W₁ (g) is the weight of the device not including the super absorbentpolymer, measured after immersing and absorbing the same into aphysiological saline solution for 30 minutes and then dehydrating thesame by using a centrifuge at 250 G for 3 minutes, and W₂ (g) is theweight of the device including the super absorbent polymer, measuredafter immersing and absorbing the super absorbent polymer into aphysiological saline solution at room temperature for 30 minutes andthen dehydrating the same by using a centrifuge at 250 G for 3 minutes.

(2) Permeability

Permeability was measured by using a 0.9% saline solution under a loadof 0.3 psi according to the method disclosed in the literature(Buchholz, F. L. and Graham, A. T., “Modern Superabsorbent PolymerTechnology,” John Wiley & Sons (1998), page 161).

More specific measurement method is described as follows, 0.2 g ofparticles having a particle size of 300 to 600 μm were taken from thesuper absorbent polymers (hereinafter, referred to as a sample) preparedin Examples and Comparative Examples, and added to a cylinder (020 mm),wherein the cylinder has a stopcock on one end, an upper limit mark anda lower limit mark thereon. The upper limit mark on the cylinder isindicated at the position of which 40 ml of (saline) solution is filledinto the cylinder, and the lower limit mark on the cylinder is indicatedat the position of which 20 ml of (saline) solution is filled into thecylinder.

50 g of 0.9% saline (NaCl) solution was added to the cylinder with thestopcock in a closed position, and left for 30 minutes. Then, ifnecessary, additional saline solution is added to the cylinder to bringthe level of saline solution to the upper limit mark on the cylinder.Then, the cylinder including the now saline-absorbed super absorbentpolymers is pressurized with a load of 0.3 psi, and left for 1 minute.Thereafter, the stopcock at the bottom of the cylinder was open tomeasure the time taken for the 0.9% saline solution to pass from theupper limit mark to the lower limit mark on the cylinder. Allmeasurements were carried out at a temperature of 24±1° C. and relativehumidity of 50±10%.

The time taken to pass from the upper limit mark to the lower limit markwas measured for respective samples (T_(s)) and also measured in theabsence of the super absorbent polymers (T₀), and permeability wascalculated by the following Equation 1:Permeability (sec)=T _(S) −T ₀  [Equation 1]

wherein:

T_(S) (unit: sec) means the time required for allowing a 0.9% saline(NaCl) solution to permeate a saline-absorbed super absorbent polymerunder a load of 0.3 psi, wherein the saline-absorbed super absorbentpolymer is prepared by swelling 0.2 g of super absorbent polymer withthe 0.9% saline solution for 30 minutes, and T₀ (unit: sec) means thetime required for allowing the 0.9% saline solution to permeate underthe load of 0.3 psi in the absence of the saline-absorbed superabsorbent polymer.

(4) Absorption Rate (Vortex Test)

50 mL of a 0.9 wt % NaCl solution was put in a 100 mL beaker, and then 2g of each super absorbent polymer prepared in Examples and ComparativeExamples was added thereto while stirring at 600 rpm using a stirrer.Then, the vortex time was calculated by measuring the amount of timeuntil a vortex of the liquid caused by the stirring disappeared and asmooth surface was formed, and the result was shown as the vortexremoval time.

(5) Fixed Height Absorption (FHA) Under Capillary Pressure

The physical property of FHA was measured and calculated as a fixedheight absorption (20 cm) by the method described in column 14/exampleof U.S. Pat. No. 7,175,910.

Preparation Example: Preparation of Base Polymer Powder

450 g of acrylic acid was added to a 2 L glass beaker, to which 693.88 gof a 24% caustic soda aqueous solution was slowly poured and mixed toprepare a first solution. At this time, neutralization heat wasgenerated, and the mixed solution was stirred at room temperature andcooled to about 41° C. Subsequently, a first solution prepared by adding0.225 g of polyethylene glycol diacrylate (PEGDA 600), 0.16 g of asurfactant (S1670) and 0.045 g of dioctyl sulfosuccinate sodium salt(AOT) to 50 g of acrylic acid; 26 g of a 4% aqueous solution of sodiumbicarbonate (NaHCO₃) (third solution); 35 g of a 0.31% aqueous solutionof ascorbic acid (fourth solution); and a solution prepared by diluting1 g of hydrogen peroxide and 0.69 g of potassium persulfate in 40 g ofdistilled water (fifth solution) were sequentially added to the firstsolution.

When the solution stirred in the beaker was gelled and stirring wasstopped, the solution was immediately poured in a Vat-type tray (15 cmin width×15 cm in length). The poured gel was foamed at about 20seconds, polymerized and slowly shrunk. The sufficiently shrunk polymerwas torn into 5 to 10 pieces and transferred into a kneader. The lid wasclosed and kneading was carried out for 5 minutes. In the kneadingprocess, the lid was opened at the lapse of 4 minutes from thebeginning, and 50 g of 3.5% aqueous solution of potassium persulfate wassprayed onto the polymer inside the kneader, and then the lid wasclosed.

Thereafter, the polymer was passed through a hole having a diameter of13 mm using a meat chopper to prepare crumbs.

Then, the crumbs were dried in an oven capable of shifting airflow upand down. The crumbs were uniformly dried by flowing hot air at 180° C.from the bottom to the top for 15 minutes and from the top to the bottomfor 15 minutes, so that the dried product had a water content of about2% or less.

The dried crumbs were pulverized using a pulverizer and classified toobtain a base polymer powder having a particle diameter of 150 to 850μm.

Example 1: Preparation of Super Absorbent Polymer

100 g of the base polymer powder obtained in Preparation Example 1 wasadded to a high speed mixer to obtain a surface crosslinking solutionwith the following composition.

The surface crosslinking solution was that prepared by mixing 3 wt % ofwater, 1 wt % of ethanol, 1 wt % of ethylene carbonate, 1 wt % ofpropylene carbonate, 0.05 wt % of a polycarboxylic acid-based copolymerdisclosed in Preparation Example 1 of Korean Patent Laid-openPublication No. 2015-0143167 (Korean Patent Application No.2014-0072343), 0.1 wt % of Na₂S₂O₅ as a thermal initiator, and 0.03 wt %of a discoloration inhibitor (trade name: Blancolen® HP), with respectto the base polymer powder.

Such surface crosslinking solution was added to the high speed mixer,and then stirred at 1000 rpm for 30 seconds. The initial temperature ofthe surface crosslinking solution immediately after the stirring was 24°C. Thereafter, the temperature raising step, the first reaction step,the temperature raising step and the second reaction step weresequentially carried out according to the conditions described in Table2 below to proceed the surface crosslinking reaction. The temperaturerising rate at each temperature raising step, the maximum reactiontemperature at the first and second reaction steps, and the reactiontime at the corresponding maximum reaction temperature are summarized inTable 2 below.

Example 2: Preparation of Super Absorbent Polymer

A surface crosslinking solution was obtained in the same manner as inExample 1, except that 1 wt % of glycerol was used instead of propylenecarbonate in Example 1.

Such surface crosslinking solution was added to the high speed mixer,and then stirred at 1000 rpm for 30 seconds. The initial temperature ofthe surface crosslinking solution immediately after the stirring was 24°C. Thereafter, the temperature raising step, the first reaction step,the temperature raising step and the second reaction step weresequentially carried out according to the conditions described in Table2 below to proceed the surface crosslinking reaction. The temperaturerising rate at each temperature raising step, the maximum reactiontemperature at the first and second reaction steps, and the reactiontime at the corresponding maximum reaction temperature are summarized inTable 2 below.

Example 3: Preparation of Super Absorbent Polymer

A surface crosslinking solution was obtained in the same manner as inExample 1, except that 1 wt % of propylene glycerol was used instead ofpropylene carbonate in Example 1.

Such surface crosslinking solution was added to the high speed mixer,and then stirred at 1000 rpm for 30 seconds. The initial temperature ofthe surface crosslinking solution immediately after the stirring was 24°C. Thereafter, the temperature raising step, the first reaction step,the temperature raising step and the second reaction step weresequentially carried out according to the conditions described in Table2 below to proceed the surface crosslinking reaction. The temperaturerising rate at each temperature raising step, the maximum reactiontemperature at the first and second reaction steps, and the reactiontime at the corresponding maximum reaction temperature are summarized inTable 2 below.

Example 4: Preparation of Super Absorbent Polymer

A surface crosslinking solution was obtained in the same manner as inExample 1, except that the content range of ethylene carbonate andpropylene carbonate in Example 1 was changed to 1.5 wt % of ethylenecarbonate and 0.5 wt % of propylene carbonate.

Such surface crosslinking solution was added to the high speed mixer,and then stirred at 1000 rpm for 30 seconds. The initial temperature ofthe surface crosslinking solution immediately after the stirring was 24°C. Thereafter, the temperature raising step, the first reaction step,the temperature raising step and the second reaction step weresequentially carried out according to the conditions described in Table2 below to proceed the surface crosslinking reaction. The temperaturerising rate at each temperature raising step, the maximum reactiontemperature at the first and second reaction steps, and the reactiontime at the corresponding maximum reaction temperature are summarized inTable 2 below.

Comparative Example 1: Preparation of Super Absorbent Polymer

A surface crosslinking solution was obtained in the same manner as inExample 1, except that only 2 wt % of ethylene carbonate was usedinstead of ethylene carbonate and propylene carbonate in Example 1.

Such surface crosslinking solution was added to the high speed mixer,and then stirred at 1000 rpm for 30 seconds. The initial temperature ofthe surface crosslinking solution immediately after the stirring was 24°C. Thereafter, the temperature raising step, the first reaction step,the temperature raising step and the second reaction step weresequentially carried out according to the conditions described in Table2 below to proceed the surface crosslinking reaction. The temperaturerising rate at each temperature raising step, the maximum reactiontemperature at the first and second reaction steps, and the reactiontime at the corresponding maximum reaction temperature are summarized inTable 2 below.

Comparative Example 2: Preparation of Super Absorbent Polymer

A surface crosslinking solution was obtained in the same manner as inExample 1, except that 2 wt % of 1,3-butanediol was used instead ofethylene carbonate and propylene carbonate in Example 1.

Such surface crosslinking solution was added to the high speed mixer,and then stirred at 1000 rpm for 30 seconds. The initial temperature ofthe surface crosslinking solution immediately after the stirring was 24°C. Thereafter, the temperature raising step, the first reaction step,the temperature raising step and the second reaction step weresequentially carried out according to the conditions described in Table2 below to proceed the surface crosslinking reaction. The temperaturerising rate at each temperature raising step, the maximum reactiontemperature at the first and second reaction steps, and the reactiontime at the corresponding maximum reaction temperature are summarized inTable 2 below.

Comparative Example 3: Preparation of Super Absorbent Polymer

A surface crosslinking solution was obtained in the same manner as inExample 1, except that 1 wt % of glycerol and 1 wt % of ethylene glycoldiglycidyl ether were used instead of ethylene carbonate and propylenecarbonate in Example 1.

Such surface crosslinking solution was added to the high speed mixer,and then stirred at 1000 rpm for 30 seconds. The initial temperature ofthe surface crosslinking solution immediately after the stirring was 24°C. Thereafter, the temperature raising step, the first reaction step,the temperature raising step and the second reaction step weresequentially carried out according to the conditions described in Table2 below to proceed the surface crosslinking reaction. The temperaturerising rate at each temperature raising step, the maximum reactiontemperature at the first and second reaction steps, and the reactiontime at the corresponding maximum reaction temperature are summarized inTable 2 below.

Comparative Example 4: Preparation of Super Absorbent Polymer

A surface crosslinking solution was obtained in the same manner as inExample 1, except that 1 wt % of 1,3-butanediol and 1 wt % of propyleneglycol were used instead of ethylene carbonate and propylene carbonatein Example 1.

Such surface crosslinking solution was added to the high speed mixer,and then stirred at 1000 rpm for 30 seconds. The initial temperature ofthe surface crosslinking solution immediately after the stirring was 24°C. Thereafter, the temperature raising step, the first reaction step,the temperature raising step and the second reaction step weresequentially carried out according to the conditions described in Table2 below to proceed the surface crosslinking reaction. The temperaturerising rate at each temperature raising step, the maximum reactiontemperature at the first and second reaction steps, and the reactiontime at the corresponding maximum reaction temperature are summarized inTable 2 below.

Comparative Example 5: Preparation of Super Absorbent Polymer

A surface crosslinking solution equal to that of Example 1 was formed.Such surface crosslinking solution was added to the high speed mixer,and then stirred at 1000 rpm for 30 seconds. The initial temperature ofthe surface crosslinking solution immediately after the stirring was 24°C. Thereafter, the temperature raising step, the first reaction step,the temperature raising step and the second reaction step weresequentially carried out according to the conditions described in Table2 below to proceed the surface crosslinking reaction. The temperaturerising rate at each temperature raising step, the maximum reactiontemperature at the first and second reaction steps, and the reactiontime at the corresponding maximum reaction temperature are summarized inTable 2 below.

The crosslinking conditions of Examples 1 to 4 and Comparative Examples1 to 5 and the ranges of the physical properties of the super absorbentpolymers obtained in those Examples and Comparative Examples arecollectively shown in Table 2 below.

TABLE 2 First reaction step Second reaction temperature TemperatureMaximum Reaction Temperature Maximum Reaction Physical properties risingrate temperature time rising rate temperature time CRC FHA PermeabilityVortex (° C./min) (° C. ) (min) (° C./min) (° C.) (min) (g/g) (g/g)(sec) (sec) Example 1 2.5 185 5 2 195 30 28.7 25.5 24 41 Example 2 2.5185 5 2 195 30 28.6 24.8 27 42 Example 3 2.5 185 5 2 195 30 28.8 25.2 2842 Example 4 2.5 185 5 2 195 30 29.1 24.9 24 40 Comparative 1 183 0 0183 40 28.5 23.1 45 42 Example 1 Comparative 1 185 0 0 185 40 29.2. 22.454 40 Example 2 Comparative 1 150 5 1 195 35 28.3 22.2 42 41 Example 3Comparative 1 185 5 1 195 35 28.6 21.6 53 42 Example 4 Comparative 1 1830 0 183 40 28.9 23.3 44 42 Example 5

Referring to Table 2, it was confirmed that the super absorbent polymersof Examples 1 to 4 exhibit more improved fixed height absorption (FHA)under pressure and liquid permeability (permeability), even whilemaintaining excellent centrifuge retention capacity (CRC) and absorptionrate (vortex).

In contrast, it was confirmed that the super absorbent polymers ofComparative Examples 1 to 4 using only one type of surface crosslinkingagent or using the outer surface crosslinking agent deviating from thesolubility parameter value range of Examples exhibit inferior absorptionunder pressure and/or liquid permeability as compared with Examples.

In addition, it was confirmed that in the case of Comparative Example 5in which the surface crosslinking is substantially carried out by asingle reaction step, the fixed height absorption under pressure and/orliquid permeability are inferior to those in the Example.

The invention claimed is:
 1. A super absorbent polymer comprising: abase polymer powder including a first crosslinked polymer, wherein thefirst crosslinked polymer polymerized from a water-soluble ethylenicallyunsaturated monomer in the presence of an internal crosslinking agent,wherein the water-soluble ethylenically unsaturated monomer having atleast partially neutralized acidic groups; and a surface crosslinkedlayer formed on the base polymer powder, wherein the surfacecrosslinking layer includes a second crosslinked polymer, wherein thesecond crosslinked polymer is formed by further crosslinking the firstcrosslinked polymer in the presence of a surface crosslinking agent,wherein the surface crosslinking agent includes at least two compounds,wherein each of the at least two compounds has a solubility parametervalue (σ) of 12.5 (cal/cm³)^(1/2) or more, wherein one of the at leasttwo compounds is a first alkylene carbonate-based compound, and anotherof the at least two compounds is selected from the group consisting of asecond alkylene carbonate-based compound and a polyhydric alcohol-basedcompound, wherein the second alkylene carbonate-based compound isdifferent from the first alkylene carbonate compound, wherein thesurface crosslinking agent further includes a polycarboxylic acid-basedpolymer in an amount of 0.01 parts to 0.5 parts by weight, based on 100parts by weight of the base polymer powder, and wherein the permeabilitymeasured and calculated by the method of the following Equation 1 is 24to 30 seconds:Permeability (sec)=T _(S) −T ₀  [Equation 1] wherein: T_(S) (unit: sec)means the time required for allowing a 0.9% saline (NaCl) solution topermeate a saline-absorbed super absorbent polymer under a load of 0.3psi, wherein the saline-absorbed super absorbent polymer is prepared byswelling 0.2 g of super absorbent polymer with the 0.9% saline solutionfor 30 minutes, and T₀ (unit: sec) means the time required for allowingthe 0.9% saline solution to permeate under the load of 0.3 psi in theabsence of the saline-absorbed super absorbent polymer.
 2. The superabsorbent polymer according to claim 1, wherein the first alkylenecarbonate-based compound includes ethylene carbonate or propylenecarbonate.
 3. The super absorbent polymer according to claim 1, whereinthe polyhydric alcohol-based compound includes ethylene glycol,propylene glycol, glycerol, polyglycerol, sorbitol or pentaerythritol.4. The super absorbent polymer according to claim 1, wherein the superabsorbent polymer has a centrifuge retention capacity (CRC) for aphysiological saline solution (0.9 wt % aqueous sodium chloridesolution) for 30 minutes of 25 g/g to 36 g/g, and a vortex removal timeof 20 to 50 seconds.
 5. The super absorbent polymer according to claim1, wherein the super absorbent polymer has a fixed height absorption(FHA) under Capillary Pressure of Loaded SAP bed of 20 to 30 g/g.
 6. Thesuper absorbent polymer according to claim 1, wherein the first alkylenecarbonate-based compound is ethylene carbonate, and wherein thepolyhydric alcohol-based compound is selected from the group consistingof propylene glycol, glycerol, polyglycerol, sorbitol andpentaerythritol.
 7. The super absorbent polymer according to claim 1,wherein the first alkylene carbonate-based compound is propylenecarbonate, and wherein the polyhydric alcohol-based compound is selectedfrom the group consisting of ethylene glycol, propylene glycerol,polyglycerol, sorbitol and pentaerythritol.